This invention relates to assay devices.
Many assay devices are designed to be xe2x80x9cself-samplingxe2x80x9d so that the user merely needs to contact the device with a liquid sample such as a body fluid in order to initiate the assay procedure. In many such devices this is the only action that the user needs to perform before the result of the assay becomes visible or readable. Many of such devices are based on the principle of xe2x80x9cimmunochromatographyxe2x80x9d in which the device contains a strip of porous carrier material along which the applied liquid sample can move. While such movement is occurring, one or more reagents within the device are carried into a detection zone on the strip and cause the assay result to be revealed. Commonly a labelled material is mobile within the strip when moist and the binding of this labelled material in the detection zone provides the means whereby the assay result becomes readable. Examples of such devices are described in EP 291 194 and EP 383 619.
The xe2x80x9cself-samplingxe2x80x9d facility can be provided by means of a bibulous sample receiving member or xe2x80x9cwickxe2x80x9d. The material from which the wick is made is chosen such that applied liquid is absorbed very rapidly into the wick. The wick then acts as a reservoir of sample liquid which feeds progressively into the porous strip to drive the immunochromatographic process. The mobile labelled reagent may be incorporated in the strip itself or elsewhere within the device, upstream from the detection zone. In EP 291 194 it is suggested that the labelled reagent can be incorporated in the wick. Ideal labelled reagents useful in these assay devices are reagents (generally specific binding reagents) labelled directly or indirectly with solid water-insoluble particulate direct labels such as dye sols, metallic (eg. gold) sols, non-metallic elemental particles such as selenium and carbon, and other minute coloured particles such as coloured latex (polystyrene) particles, all known per se for this purpose.
The present invention provides an improved assay device in which the wick comprises non-woven fabric.
Preferred non-woven material is made by the process known as hydroentanglement. Ideally no chemical treatment or chemical curing is used during the manufacturing process. Such materials are already known per se and are widely used to make cleaning cloths and wipes.
For the purposes of the invention the non-woven wick material is preferably hydrophilic. If the overall character of the fabric is hydrophobic, it can be treated for example with surface active agent to render it hydrophilic in use. It will usually be made from a blend of fibres. This blend can be a mixture of hydrophobic fibres and hydrophilic fibres, but the overall character of the material preferably hydrophilic. An ideal blend comprises a mixture of viscose and polyester. Preferably, the blend is about 30% viscose and about 70% polyester.
An important preferred aspect of the invention is that the wick should comprise only a single sheet or layer of the non-woven fabric. This considerably facilitates manufacture and quality control of an assay device when the wick is used to contain one or more reagents important to the assay chemistry, such as a mobile labelled reagent. If the wick is constructed of multiple layers of bibulous material, it is difficult to ensure that the reagent(s) are deposited consistently in the wick during manufacture, and flow of sample liquid through the multi-layer structure may be uneven, and lead to inefficient or variable uptake of the reagent(s).
In order to constitute an effective wick when used in an immunochromatographic assay device, the wick should have sufficient absorptive capacity. The wick liquid capacity should exceed the capacity of the strip (together with any sink at the distal end of the strip, if provided). The xe2x80x9cweightxe2x80x9d of the non-woven material is important. The weight of the material is preferably at least about 50 g/m2, and more preferably at least about 70 g/m2. Generally it is not necessary for the weight to exceed about 120 g/m2.
Preferably, the non-woven fabric layer is bonded to a supporting layer of non-water-absorbent material, such as plastics sheet. Polyester sheet is ideal. Bonding can readily be achieved using a variety of adhesives, known per se in the lamination art, the adhesion step being induced by pressure, heat or the use of two-component adhesives. It is self-evident that the quantity and nature of the adhesive should not significantly impair the absorbency and flow properties of the non-woven fabric when bonded to the support. Neither should the adhesive contain any reagents, such as unreacted excess monomers, in amounts that could interfere with the efficiency of the specific binding or other reactions that must occur within the assay device during use.
Although the fabric materials used in this invention are conventionally described as being xe2x80x9cnon-wovenxe2x80x9d, this does not necessarily mean that the fibres that make up such fabric are arranged in a totally random manner. It is generally found, as a result of the process by which the fabric is made, that a distinct proportion of the fibres lie predominately in one direction and that the remainder lie predominately in a direction at right angles to the first direction. In constructing a device in accordance with the invention, the non-woven fabric is preferably selected and arranged such that the majority of the fibres lie parallel to the direction in which liquid should flow along the wick into the device. Preferably, the numerical ratio of flow-parallel fibres to flow-orthogonal fibres should be about 2:1 or greater, provided that there are sufficient flow-orthogonal fibres present to maintain the mechanical integrity of the non-woven material for manufacturing purposes.
If desired, the wick can incorporate components that assist the performance of the assay, such as buffering agents and surfactants.
Use of a wick made from non-woven fabric material of the preferred weight as described above, leads to much improved wicking properties and to very efficient release of any dried labelled reagent which may be incorporated in the wick.